The European Space Agency probe Rosetta will pass two asteroids named Lutetia and Steins on
its journey to comet 67P/Churyumov-Gerasimenko.

Image credit: ESA/AOES Medialab

The Rosetta Science Working Team has made the final selection of the asteroids that Rosetta
will observe at close quarters during its journey to Comet 67P/Churyumov-Gerasimenko. Steins
and Lutetia lie in the asteroid belt between the orbits of Mars and Jupiter.

Rosetta's scientific goals always included the possibility of studying one or more asteroids
from close range. However, only after Rosetta's launch and its insertion into interplanetary
orbit could the ESA mission managers assess how much fuel was actually available for fly-bys.
Information from the European Space Operations Centre (ESOC) in Germany enabled Rosetta's
Science Working Team to select a pair of asteroids of high scientific interest, well within
the fuel budget.

The selection of these two excellent targets was made possible by the high accuracy with
which the Ariane 5 delivered the spacecraft into its orbit. This of course leaves sufficient
fuel for the core part of the mission, orbiting Comet 67P/Churyumov-Gerasimenko for 17 months
when Rosetta reaches its target in 2014.

Asteroids are primitive building blocks of the Solar System, left over from the time of its
formation about 4600 million years ago. Only a few asteroids have so far been observed from
nearby. They are very different in shape and size, ranging from a few kilometres to over 100
kilometres across, and in their composition.

The targets selected for Rosetta, Steins and Lutetia, have rather different properties.
Steins is relatively small, with a diameter of a few kilometres, and will be visited by Rosetta
on 5 September 2008 at a distance of just over 1700 kilometres. This encounter will take place
at a relatively low speed of about 9 kilometres per second during Rosetta's first excursion into
the asteroid belt.

Lutetia is a much bigger object, about 100 kilometres in diameter. Rosetta will pass within
about 3000 kilometres on 10 July 2010 at a speed of 15 kilometres per second. This will be during
Rosetta's second passage through the asteroid belt.

Rosetta will obtain spectacular images as it flies by these primordial rocks. Its onboard
instruments will provide information on the mass and density of the asteroids, thus telling us
more about their composition, and will also measure their subsurface temperature and look for gas
and dust around them.

Rosetta began its journey on 2 March, and is well on its way. Commissioning of its instruments
has already started and is proceeding according to plan.

"Comets and asteroids are the building blocks of our Earth and the other planets in the Solar System.
Rosetta will conduct the most thorough analysis so far of three of these objects," said Prof. David
Southwood, Director of ESA's Science Programme. "Rosetta will face lots of challenges during its
12-year journey, but the scientific insights that we will gain into the origin of the Solar System
and, possibly, of life are more than rewarding."

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